Premier Pet
Scent-Free Aspen Bedding.
I got it from PetSmart. I also have some little apple tree branches in there that they seem to really like.

If you look really close you can see the little pieces that came off a pringle can as well. They go nuts over pringle cans! ha I give them something new every week so that they keep their minds off trying to escape. I haven't had any chewing at the tub yet. (Knock on wood)